1. Field of the Invention
The present invention relates to a light scanning device and an image forming apparatus provided with the same.
2. Description of the Background Art
Conventionally, a light scanning device (LSU: laser scanner unit) provided in an image forming apparatus includes a reflecting mirror (mirror member) for reflecting light, emitted from a light source and then reflected by a polygon mirror, toward a photosensitive drum which is an object to be scanned.
The aforementioned reflecting mirror has a rectangular mirror surface which is elongate in a horizontal direction, the reflecting mirror being supported by a housing at both ends of the mirror in a longitudinal direction thereof. For this reason, vibration of the polygon mirror attached also to the housing is transmitted to the reflecting mirror through the housing. If the natural frequency of the reflecting mirror generally matches the frequency of vibration of the polygon mirror produced by rotary motion thereof, causing so-called resonance in which the vibration of the reflecting mirror is amplified resulting in yet greater vibration of the reflecting mirror.
One problem which can occur due to the resonance of the reflecting mirror produced in the aforementioned manner is that the light would not be reflected exactly onto the photosensitive drum, resulting in deterioration in image quality.
Under such circumstances, Japanese Laid-open Patent Publication No. 2002-267978 introduces an arrangement for preventing the aforementioned resonance problem by means of a single piece of a metal member (resonance preventing member) which is attached to a reflecting mirror, covering generally an entire longitudinal range thereof, to vary the weight and natural frequency of the reflecting mirror.
Generally, the reflecting mirror has an extremely flat surface whose flatness is sufficiently higher than that of the resonance preventing member. Therefore, in the arrangement of Japanese Laid-open Patent Publication No. 2002-267978 in which the resonance preventing member is attached to the reflecting mirror to cover generally the entire longitudinal range thereof, the flatness of the reflecting mirror is greatly affected by the flatness of the resonance preventing member and, thus, the reflecting mirror warps along the longitudinal direction due to warpage of the resonance preventing member along the longitudinal direction thereof. This produces a problem that deterioration in image quality occurs due to a reduction in the flatness of the reflecting mirror and resultant nonlinear scanning of light over the photosensitive drum.